All biological systems are subject to stochastic variation. In mammals this is obvious from the large individual variation in life span and patterns of aging-related pathology, even in genetically homogeneous animals. Stochasticity is also apparent at the molecular level. Random molecular fluctuations creating variability in gene expression within a cell population have been demonstrated in bacteria and yeast. To some extent this is inherent to the nature of the processes of information transfer, especially at small numbers of mRNA or protein products per cell. However, noise at the molecular level can also have external causes, varying from random damage to the genome to variability in regulatory signals. While sometimes advantageous, i.e., in development and evolution, increased Stochasticity in aging is generally viewed as having detrimental effects on cellular function. The central hypothesis in this proposal is that oxidative stress, a likely cause of aging, increases stochastic variability of gene expression, that it does so by causing both genetic and epigenetic changes in cells, and that cells and organisms possess a variety of genetic pathways and cellular responses to mitigate or buffer against unduly large stochastic changes. We will test this hypothesis in two specific aims. First, we will comparatively analyze four different model systems of aging, nematodes, fruit flies, mice and human cells, for mutation accumulation at a similar lacZ reporter construct. We will also investigate how such genome level Stochasticity depends on genetic factors known to cause aging-related neurodegenerative disease, how it differs between human and mouse cells and how it can be modulated by genetic factors. Second, we will directly measure transcriptional noise levels in mouse neurons and neuronal stem cells during aging and in model systems for human neurodegenerative diseases. In parallel, we will study similar transcriptional noise in human and mouse fibroblasts in different genetic backgrounds and among individual nematodes during aging. We expect that the proposed study will provide a new dimension to existing paradigms in the field by defining the role of Stochasticity in aging phenotypes and identifying the genetic and biochemical mechanisms that influence it.